In December 1992 Science magazine named NO the molecule of the year. In the atmosphere nitric oxide is a noxious chemical but in the biological system in controlled small doses it is extraordinarily beneficial. It helps to maintain blood pressure, dilate blood vessels, help kill foreign invaders in the immune response and is a major biochemical mediator of penile erection and probably a biochemical component of long term memory. These are just a few of the many roles which are just beginning to be discovered and have been documented in the scientific literature in the last 5 years.
NO is synthesized from the amino acid L-arginine by the enzyme NO synthase. The release of NO via this enzyme has other pathological or biological consequences including pathological vasodilation and tissue damage. The formation of NO by this enzyme in vascular endothelial cells opened up what can be considered a new era of biomedical and clinical application research. NO released from the endothelial cells is indistinguishable from EDRF (The endothelial derived relaxation factor) in terms of biological activity, stability and susceptibility to inhibitors and promoters. There is available evidence indicating that the cardiovascular system is in a state of constant active vasodilation depending on the generation of NO. Indeed NO can be considered the endogenous vasodilator. Decreased synthesis of NO may contribute to the origin of conditions such as atherosclerosis and hypertension. Nitro vasodilators that have been clinically used the last 100 years and are still widely used in angina pectoris, congestive heart failure, hypertensive emergencies, pulmonary hypertension, etc. are acting through the formation of NO.
Biochemical experimentation has shown that the nitro vasodilator and NO act by activating the soluble guanylate cyclase and it is widely accepted that this activation and the consequent increase in cyclic GMP (guanosine-5',5'-cyclic phosphoric acid) levels induces a sequence of protein phosphorylation associated with smooth muscle relaxation. Nitrovasodilators also generate NO in non-enzymatic reaction, and this leads to the stimulation of soluble guanylate cyclase. The metabolism of nitroglycerin, for example and other organic nitrates by denitration leads to the formation of nitrites which subsequently undergoes biotransformation to generate vasoactive intermediates such as S-nitroso-thiols and nitric oxide. Direct evidence has been established for nitric oxide formation from GTN (glyceryl trinitrate, nitroglycerin) during incubation with intact bovine pulmonary artery. This transformation has been measured chemically and is dependent on incubation time. Nitric oxide provokes vaso-dilatation, and inhibits platelet aggregation. It is involved in increased cerebral cortical blood flow, following the stimulation of nucleus basalis of Heynert in anasthetized rats. Endogenous NO acts as a mediator of gastric mucosal vaso-dilatation. It does not directly modulate the acid secretory response but makes a substantial contribution to the mucosal vaso-dilatation associated with the stimulation of gastric acid secretion. It also mediates estrogen-induced increases in uterine blood flow. The observed vaso-dilatation can be antagonized by the intra-arterial administration of nitric synthase inhibitor L-nitro arginine methyl ester.
Inhaled NO is a selective pulmonary vaso-dilator that can prevent thromboxane-induced pulmonary hypertension during the heparin protamine reaction and does so without causing systemic vasodilation. In biological media NO is very active in a very small concentration. It reacts with O.sub.2 to produce NO.sub.2 which then forms NO.sub.2 - and NO.sub.3 - in neutral aqueous solutions according to the following equations: EQU 2NO+O.sub.2 .fwdarw.2NO.sub.2 I
then EQU 2NO.sub.2 +H.sub.2 O.fwdarw.NO.sub.2 -+2H.sup.+ +NO-.sub.3.II
Neither NO-.sub.2 nor NO-.sub.3 as their sodium salts cause any reaction to vascular strips at a concentration below 10 uMol. The exact physiological stimuli for the generation of NO are not fully understood but the pulsatile flow and shear stress seem to be the main determinants.
NO has a great potential to be applied and used clinically in the treatment of a variety of diseases and can offer a better substitute for a wide range of commercially available vasoactive drugs. This could be realized only if a suitable delivery system is designed for NO administration. Today these systems do not exist and there is a need for the development of these systems. All of the results that have been generated so far were obtained indirectly either by working with NO synthase inhibitors or by delivering NO as Prodrug (organic nitric oxide donors). The authentic bioactive molecule has never been formulated in a convenient delivery system having the capability of targeting and preserving the integrity of nitric oxide molecule. The development and the optimization of such systems is of paramount significance for the use of NO in therapeutics.
Nitric oxide NO, is a colorless gas with a boiling point of -151.7.degree. C., melting point of -163.degree. C. and density of the liquid of 1.269 at boiling point. The NO molecule contains an odd number of electrons. It is soluble in water, 4.7 parts per 100 parts volume per volume at 20.degree. C. and 1 atmosphere. NO reacts readily with oxygen to form brown ntirogen dioxide. Under physiological conditions, nitric oxide can be interconverted to different redox forms with different distinctive chemistries. These forms are NO, NO.sup.+ and NO.sup.-. Current research indicates that nitrovasodilators carry out their physiological functions by releasing NO (neutral nitric oxide). It is to be noted that the biological half life of this active molecule depends critically on the concentration of the reactants used and on the initial concentration of nitric oxide formed.